With development of mobile communications technologies, LTE (Long Term Evolution, Long Term Evolution) keeps improving data transmission capabilities and data transmission speeds of wireless networks by means of new technologies and modulation methods. To improve a data transmission rate of UE, LTE currently supports a CA (Carrier Aggregation, carrier aggregation) technology. In the CA technology, specifically, an access network device configures multiple carriers for UE that supports carrier aggregation, to perform uplink communication and downlink communication, to enable the UE to support transmission at a higher data rate.
When CA is being performed, an access network device sends multiple synchronous carriers to UE, and the UE detects a PDCCH (Physical Downlink Control Channel, physical downlink control channel) of each carrier, where the PDCCH carries scheduling information of a PDSCH (Physical Downlink Shared Channel, physical downlink shared channel). After the UE receives the PDSCH, if decoding is correct, the UE feeds back an ACK (ACKnowledge, acknowledgment) to the access network device on a PUCCH (Physical Uplink Control Channel, physical uplink control channel), and if decoding is incorrect, the UE feeds back a NACK (NACKnowledge, non-acknowledgment) to the access network device on the PUCCH. When the ACK/NACK is fed back to the access network device, an ACK/NACK codebook needs to be generated. The ACK/NACK codebook is a bit stream of the ACK/NACK before coding being arranged according to a particular order. For an ACK/NACK bit corresponding to a downlink subframe of the PDSCH actually received by the UE, the ACK or NACK may be set according to a feedback of an ACK or a NACK for correct or incorrect reception. For an ACK/NACK bit corresponding to a downlink subframe that is not scheduled or not received, a NACK needs to be padded.
However, with continuous development of LTE technologies, a relatively large quantity of carriers may need to be configured for UE, but there are not necessarily many carriers that are actually scheduled by the UE. An example in which there are two mainstream TDD uplink/downlink configurations in a current network is used. An uplink subframe 2 of one carrier supports a feedback of 4 ACK/NACK bits. Therefore, an uplink subframe 2 for which 32 carriers are configured supports a feedback of 128 ACK/NACK bits. However, the UE may actually schedule only 10 carriers, that is, a downlink subframe actually scheduled by the UE corresponds to 40 ACK/NACK bits. During generation of an ACK/NACK codebook, NACK padding needs to be performed for 128−40=88 ACK/NACK bits that correspond to a downlink subframe that is not scheduled; further the generated ACK/NACK codebook needs to be sent to an access network device. It can be learnt that in a manner of using a currently determined ACK/NACK codebook, a large amount of NACK padding occurs, occupying a relatively large quantity of PUCCH resources, and resulting in low uplink resource utilization.